The present invention relates to combustion products detectors and alarms and, in particular, to battery voltage monitor circuits for such detectors.
Most combustion products detectors are battery-powered to prevent the unit from being disabled in the event of an AC power failure. Both because of the small drain placed on the battery in maintaining the combustion products detector in a ready condition, and because of the limited shelf life of storage batteries, the energy level of the battery will gradually decrease over time. Accordingly, it is important that the user be able to monitor the energy level of the battery so as to be alerted when the energy level approaches a level beneath which it will be insufficient to power the combustion products detector since, at or before that level, the battery must be replaced if the combustion products detector is to remain operative.
Battery energy level monitoring devices are well-known in the art, one such device which monitors the output voltage of the battery in a combustion products detector being disclosed in U.S. Pat. No. 4,004,288. The difficulty with such prior art battery voltage monitoring circuits is that they will faithfully generate an audible alarm signal whenever the output voltage of the battery reaches the trouble level, and will continually generate the signal until the battery is replaced or until its output voltage becomes insufficient to power the battery voltage monitoring circuit. When the battery voltage alarm signal occurs in the middle of the night, it can be quite inconvenient and annoying. Sometimes the annoyance is such that the user will disable the system to silence it, and then forget that he has done so, thereby rendering the system useless.